From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f68.google.com ([209.85.218.68]:38483 "EHLO mail-oi0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726821AbeILUed (ORCPT ); Wed, 12 Sep 2018 16:34:33 -0400 Received: by mail-oi0-f68.google.com with SMTP id x197-v6so4533295oix.5 for ; Wed, 12 Sep 2018 08:29:31 -0700 (PDT) MIME-Version: 1.0 References: <20180911183909.233413-1-jannh@google.com> In-Reply-To: <20180911183909.233413-1-jannh@google.com> From: Jann Horn Date: Wed, 12 Sep 2018 17:29:04 +0200 Message-ID: Subject: Re: [PATCH] proc: restrict kernel stack dumps to root To: Alexey Dobriyan Cc: Ken Chen , kernel list , linux-fsdevel@vger.kernel.org, Will Deacon , Laura Abbott , Andy Lutomirski , security@kernel.org, Catalin Marinas , Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Linux API Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: +linux-api, I guess On Tue, Sep 11, 2018 at 8:39 PM Jann Horn wrote: > > Restrict the ability to inspect kernel stacks of arbitrary tasks to root > in order to prevent a local attacker from exploiting racy stack unwinding > to leak kernel task stack contents. > See the added comment for a longer rationale. > > There don't seem to be any users of this userspace API that can't > gracefully bail out if reading from the file fails. Therefore, I believe > that this change is unlikely to break things. > In the case that this patch does end up needing a revert, the next-best > solution might be to fake a single-entry stack based on wchan. > > Fixes: 2ec220e27f50 ("proc: add /proc/*/stack") > Cc: stable@vger.kernel.org > Signed-off-by: Jann Horn > --- > fs/proc/base.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index ccf86f16d9f0..7e9f07bf260d 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns, > unsigned long *entries; > int err; > > + /* > + * The ability to racily run the kernel stack unwinder on a running task > + * and then observe the unwinder output is scary; while it is useful for > + * debugging kernel issues, it can also allow an attacker to leak kernel > + * stack contents. > + * Doing this in a manner that is at least safe from races would require > + * some work to ensure that the remote task can not be scheduled; and > + * even then, this would still expose the unwinder as local attack > + * surface. > + * Therefore, this interface is restricted to root. > + */ > + if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN)) > + return -EACCES; > + > entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries), > GFP_KERNEL); > if (!entries) > -- > 2.19.0.rc2.392.g5ba43deb5a-goog >